Fix dart2js warnings and add test to ensure it stays clean.

pkg/front_end/test/fasta/testing.json

Index: pkg/front_end/test/fasta/testing.json
diff --git a/pkg/front_end/test/fasta/testing.json b/pkg/front_end/test/fasta/testing.json
index 5e5583123c8b1e14b507815d814a3e8737ed48b2..2e9c0734a4c6e5a5df4e706a1e329cc40631e155 100644
--- a/pkg/front_end/test/fasta/testing.json
+++ b/pkg/front_end/test/fasta/testing.json
@@ -153,8 +153,6 @@
       "outline_test.dart",
       "sdk_test.dart",
       "../../../analyzer/lib/src/fasta",
-      "../../../compiler/lib/src/parser",
-      "../../../compiler/lib/src/scanner",
       "../../../../utils/kernel-service/kernel-service.dart",
       "../../../../tools/patch_sdk.dart"
     ],
@@ -163,8 +161,6 @@
       "/third_party/pkg/",
       "/third_party/pkg_tested/",
       "/pkg/analyzer/(?!.*/(fasta|kernel)/)",
-      "/pkg/compiler/lib/src/(?!(scanner|parser)/)",
-      "/pkg/compiler/lib/src/parser/partial_elements\\.dart$",
       "/pkg/js_ast/"
     ]
   }